1. Field of the Invention
This invention relates to a surface smoothing method for smoothing the surface of an object such as substrate with a semiconductor film or insulating film formed thereon, and a method of forming an SOI substrate using the surface smoothing method.
2. Description of the Prior Art
To attain high density and high integration of a semiconductor circuit device, the development of a three-dimensional semiconductor circuit device of laminated structure is being promoted, in which the semiconductor devices are incorporated into each semiconductor thin film which is laminated alternately with an insulating film. When manufacturing such three-dimensional semiconductor circuit devices, however, minute irregularities can be produced on the surface of each semiconductor thin film or each insulating film in the intermediate stage of the process. Since such irregularities, if uncorrected, can exert a negative influence upon the quality of the film grown in the process, the surface of the growing film must be smoothed free of irregularities in the process.
The necessity of surface smoothing will now be particularly described with an example of an SOI (Silicon On Insulator) substrate comprising a single crystal insulating film formed on a substrate with a single crystal silicon film epitaxially grown thereon.
The SOI substrate is considered an elementary material for obtaining integrated circuits of excellent characteristics. Particularly, the SOS (Silicon On Sapphire) substrate using sapphire has excellent characteristics such as higher speed owing to decreased floating capacity and higher integration owing to a decreased separating area between elements.
Nevertheless, for the SOS substrate, sapphire is expensive and larger sapphire is unavailable. Therefore, research and development of SOI substrate forming techniques by use of single crystal magnesia spinel (MgO.multidot.Al .sub.2 O .sub.3) has gained attention recently.
One well known technique of forming single crystal magnesia spinel film on a single crystal silicon substrate is the vapor-phase epitaxial growth method (hereinafter called VPE method) as described, for example, in Japanese Patent Publication No. 55119-1983 and Japanese Patent Publication No. 55120-1983.
In Extended Abstracts of the 15th Conference on Solid State Devices and Materials, Tokyo, 1983, pages 31-34, is reported an improvement of the gate insulating characteristic of and integrated circuit by the following method: after forming a single crystal magnesia spinel film 2 on a single crystal silicon substrate 1 by the VPE method of MgCl.sub.2 --Al--HCl--CO.sub.2 --H.sub.2 --N.sub.2 series, anneal it at 1100.degree. C. in an oxygen atmosphere to form oxidized silicon film 3 by the oxidization of the interface with the magnesia spinel film of the silicon substrate 1. Then, make a single crystal silicon film 4 epitaxially grown on the magnesia spinel film 2. Thus, with the double gate insulating films of the magnesia spinel film 2 and oxidized silicon film 3 formed, improve the dielectric separation characteristic between the silicon substrate 1 and the upper silicon film 4; by using such an SOI substrate, having double gate, improved insulating characteristics of the integrated circuit are achieved.
The change of the crystallinity of the magnesia spinel film 2 when annealed at a high temperature in an oxygen atmosphere about 2 hr. after being formed on the silicon substrate 1 by the VPE method, as described above, is studied by X-ray diffractometry, giving a result as shown in FIG. 2.
FIG. 2 shows the ratio of the X-ray diffraction intensity of (400) magnesia spinel annealed at various temperatures to that of (400) magnesia spinel before annealing. A higher ratio indicates increased crystallinity of the magnesia spinel film 2, and as obvious from FIG. 2, the higher the annealing temperature, the better is the crystallinity of the magnesia spinel film 2.
However, annealing at 1,200.degree. C. roughens the surface of the magnesia spinel film 2, worsening its flatness, as shown by the electron micrograph in FIG. 3, although improving its crystallinity. In addition, the reflected high energy electron beam diffraction pattern in FIG. 4, showing the surface crystalline structure of the magnesia spinel film 2, fails to exhibit an elongated pattern, and the conditions and crystalline structure of the surface of the magnesia spinel film 2 when annealed at 1200.degree. C. are poor. Therefore, the quality of the silicon film 4 epitaxially grown on the magnesia spinel film 2 may be poor.
On the other hand, when annealing at a temperature of 1100.degree. C. or lower, the roughening of the surface of the magnesia spinel film 2 is decreased, but the increase of the crystallinity of the magnesia spinel film 2 is remarkably small in comparison with annealing at 1200.degree. C., as shown in FIG. 2. Thus, the quality of the silicon film 4 epitaxially grown on the magnesia spinel film 2 is not satisfactory.
Therefore, mirror polishing is commonly given to the substrate surface to make it smooth before the process for forming the insulating film or the semiconductor thin film.
However, as a technique for smoothing the surface of the grown film during the process, no effective method has been available so far other than that described above. In the case of a polishing method, the surface of the grown film can be contaminated with the abrasive and a mechanical distortion or a peeling of a film according to the hardness and adhesive strength of the grown film can be caused. In addition, every time polishing is performed, the process must be interrupted to set the wafer to the polishing device, requiring much labor.